As mentioned in a response post, plant weights are a good solution. I have not been able to find them to buy - even the guys at my LFS can't tell me where to purchase them. But usually if you buy established plants (unpotted) when they are bagged up, you might see if the weights are on the bottoms of the stems, if so, ask if you can have them! Most LFS are ok with giving them to you, as they most likely have a ton!
I've also used fishing line and other fine thread to attach a plant to a bit of driftwood and that works great too.
Good luck! Planted tanks can sometimes be a chore, but they can be so worth it!